Test login API to know number of supported request per second?

I write an login API and want to test it, i want my API support up to 500 concurrent login request (with different credential). How can I know the number of concurrent login requests my API can support. I have tried using jmeter with
+ thread = 500
+ ramp up = 1 (second)
+ count loop = 3
and I see the result in summary listener throughput around 330/s and the other information. What does it mean?
Thank you a lot for any suggestions.